Body weight regulation involves adjustments in food intake that compensate for fluctuations in energy stores. These adjustments are comprised of alterations in meal number, size, or both. Compared with the well understood systems that regulate meal termination, and thus, meal size, those that regulate meal initiation and frequency remain enigmatic. The recently discovered enteric hormone, ghrelin, is implicated in the cascade of events leading to meal initiation, but evidence favoring this assertion is largely circumstantial. We propose to test critically the requirement for ghrelin and its most well established CNS target - NPY/Agrp neurons - in meal initiation, using loss-of-function experiments. We will address the following broad questions. (1) Is ghrelin signaling required for normal meal initiation, meal termination, or both? Using state-of-the art equipment to measure food intake continuously on a second-to-second basis, we will assess the effect on meal number, size, and duration, of novel, highly potent antagonists of the ghrelin receptor (GHS-R), at doses that decrease overall food intake. Experiments will be conducted on ad libitum-fed mice as well as those subjected to a variety of acute and chronic states of energy deficit that challenge meal-initiation signals. We will study wild-type and GHS-R /- mice in parallel, both to verify that the anorectic effects of GHS-R antagonists result specifically from blockade of the GHS-R, and also to compare the effects of complementary pharmacologic and genetic ablation of GHS-R signaling. (2) Are NPY/Agrp neurons required for normal meal initiation, energy homeostasis, and response to ghrelin? Our co-investigator, Dr. Greg Barsh, has created mice in which NPY/Agrp neurons are gradually and continuously destroyed in adulthood. The animals are a valuable resource to determine the requirement for NPY/Agrp neurons in each of ghrelin's known anabolic actions, as well as to assess the role of NPY/Agrp neurons themselves in meal patterning and overall energy homeostasis. We propose a thorough phenotyping analysis of these mice to address these issues. (3) Do intracellular signaling events triggered by ghrelin in key hypothalamic neurons - especially NPY/Agrp neurons - oppose those triggered by leptin and insulin (and vice versa)? Together, these studies have the potential to fundamentally advance our understanding of feeding biology at the molecular, cellular, and behavioral level.